Magazine Clippings
Cut up words from a magazine and throw them into a hat/bowl/receptacle of your choice. Without looking, pick five and use as many of them as you can in a story.
Extra challenge: Use them all in your title or first line.
Let Fate Decide
Shuffle a tarot deck and pick the first three cards that come up. Build a story around them.
Haven't got a tarot deck? Then use a free online tarot reading. The "Three Fates" spread will pick 3 cards for you. You could try the Rider Waite deck, which is rich in symbolism, or really leave it up to fate and select "Choose for me".
Warning: If you do play around with the online site, remember to actually do the writing.
Come to Your Senses
Write an entire scene using a sense other than sight to describe what is happening.
Picture Seeds
Use one (or if you're showing off, all) of the pictures below to inspire you. Write for 15 minutes.